Electroacoustic transducer



Sept. 4, 1951 E. E, M01-T 2,566,849

ELECTROACOUSTIC TRANSDUCER Filed 0013. 19. 1946 /6 /44 'y 2x0 F IG. l

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/NVENTOR E. E. MOTT A TTORNEV Patented Sept. 4, 1951 UNITED STATES PATENT oFElcE ELECTROACOUSTIC TRANSDUCER.

Edward E. Mott, Upper. Montclair, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a, corporation of New York Application October 19, 1946, Serial No. 704,483

3 Claims.`

This invention relates to electroacoustic` transducer and more particularly to such transducers of the ring armature type such as disclosed in Patent 2,249,160, granted July l5, 1941 to Edward E. Mott.

One object of this invention is to simplify the construction of electroacoustic transducers of the ring armature type and, thus, to facilitate the manufacture of such devices in quantity and at relatively low cost.

Another object of this invention is to improve the performance of such transducers.

Electroacoustic transducers of the ring armature type comprise, in general, an annular magnetic system including a ring armature to which a lightweight diaphragm member is coupled. 'The magnetic system includes also a permanent magnet which advantageously, in order to reduce the Weightfand size of the transducer and to provide the large polarizing flux requisite for efficient and high power translation of signals, is made of a high magnetic strength alloy, such as Alnico." Such alloys are extremely hard and difficult to machine.

The performance of a ring armature type electroacoustic transducer is dependent very largely upon the air-gap between the armature and the pole-piece in juxtaposition thereto. In order to realize high eiiiciency operation for any unit, this air-gap must be small and accurately meet preassigned dimensions; in order that similar devices may have the same operating characteristics, it is necessary that this air-gap be essentially the same in the several devices.

In accordance with one feature of this invention, the requisite small and accurate air-gap in any transducer and the substantial identity of the air-gaps in a multiplicity of transducers are attained facilely and with minimum working of the magnet. More specifically, in accordance with one feature of this invention, the inner polepiece is provided with a ange remote from the pole tip thereof, and the magnet is seated upon the iiange and serves as the outer pole-piece. One end face of the magnet is made accurately coplanar with the pole tip of the inner pole-piece and the armature is seated upon `a spacer in turn seated upon the end face of the magnet. The air-gap between the armature and inner pole-piece is xed by the thickness of the spacer. Thus, the air-gap of desired length is realized by mere association of the armature, spacer and pole-piece magnet assembly.

As disclosed in the Mott patent above-identified, improved perfomance may be realized by i providing an auxiliary magnet opposite the1armature and the pole faces of the main magnetic structure. In accordance with another feature of this invention. a low reluctance path iis provided between the auxiliary magnet and the main magnetic structure whereby the magnetic etliciency of the transducer is increased. i

The response level of the transducer is de pendent to a large extent upon the vibrational parameters of the diaphragm. In accordance with a further feature of this inventionjf` a low mass, bodily vibratile diaphragm membershaving low internal losses and substantially unaffected by ambient temperature variations or humidity is realized. More specifically, in accordance with this further feature of the invention, this diaphragm member is formed of a synthetic'jplastic fibre cloth impregnated with a phenolic condensation product. ff

In accordance with still another feature'of this` invention. an auxiliary signal coil coupled to the alternating current flux path of the niagnetic system is provided in series with the maii signal coil, thereby to increase the linkage to the signal 1 flux and effect an increased force factor for the transducer.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing which:

Fig. l is a cross-sectional View of an electroacoustic transducer illustrative of one embodiment of this invention;

Fig. 2 is a fragmentary sectional view of a portion of the transducer illustrated in Fig. l showing the paths for the biasing or polarizing; and signal fluxes in the magnetic system;

Fig. 3 is a View in cross-section of a transducer illustrative of another embodiment of this invention;

Fig. 4 is a plan view taken along plane 4--4 of Fig. 3;

Fig. 5 is a plan view of one form of diaphragm and armature assembly which may be employed in a modification of the transducers illustrated in Figs. 1 and 3; I

Figf' is a sectional view taken along plane G-B of Fig. 5; and y Fig. 7 is a plan view of a modification of the armature included in the transducers illustrated in Figs. 1 and 3.

Referring now to the drawing, the transducer illustrated in Figs. l and 2, which may be utilized either for translating sound into electrical signals upon assembly oi the device. Thus, the several parts of the device may be taken from stock and assembled facilely to produce in quantity transducers having essentially the same operating characteristics.

In a modification, illustrated in Figs. 3 and 4, a plurality of individual, equally spaced pole-pieces 29 may be employed in place of the single polepiece l0, I I in the transducer illustrated in Fig. 1. Each pole-piece has thereon a signal coil 30; the several coils may be connected in series aiding or in parallel. Also, as shown in Figs. 3 and 4, the plate 2| may be provided with apertures 3l having acoustic resistance material 32 extending thereacross. These apertures, together with the chamber between diaphragm l5 and plate 2|, constitute an acoustic network comprising mass, stiiness and resistance, which functions to reduce the response peak due to the diaphragm resonance and to enhance the response throughout a band of frequencies, whereby a substantially uniform response over a wide frequency range is obtained. It will be understood that such a network may be incorporated also in devices of the construction illustrated in Fig. 1.

The response of a transducer of the type i1- lustrated, operating either as a receiver or a transmitter, is directly dependent upon the amplitude of the diaphragm vibrations. Enhanced vibration may be obtained by employing a diaphragm-armature assembly of the construction illustrated in Figs. 5 and 6. As shown in these figures, the diaphragm comprises a central dish- -ed portion 33 and an integral, annular peripheral portion 34 provided with stiffening flutes or ribs 35. The armature is aiiixed to the diaphragm portion 34 and, as shown in Fig. 6, is of less Width than this portion. The diaphragmarmature assembly may be mounted upon the spacer I3 in the same manner as the assemblies illustrated in Figs. 1 and 3. The diaphragm portion 34, it will be noted, constitutes a lever whereby the amplitude of displacement of the diaphragm portion 33 in response to vibration of the armature is increased.

The response of a transducer is dependent also upon the stiffness of the armature I4. Magnetic considerations restrict the thinness of armature that can be employed to best advantage. However, the radial width of the armature can be varied substantially without substantial degradation of the operating efficiency, thereby to adjust the armature stiffness to a desired value and x the resonant frequency of the vibrating system.

Changes in armature stiffness by varying the radial width of the armature are accompanied, for a given armature thickness, by changes in the armature mass; For a given armature thickness and width, the armature stiffness may be reduced substantially by providing the armature with radial slots 36 asillustrated in Fig. 7.

Although Yspecific embodiments of the invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

pole face, a magnetic spacer seated upon the other end of said magnet, said armature having its peripheral portion seated upon said spacer and being held thereon solely by magnetic attraction, an annular magnetic member overlying said armature and having its inner and outer marginal portions opposite said pole face and said other end of said magnet respectively, and an annular low reluctance member spacing said magnetic member from said spacer.

2. An electroacoustic transducer in accordance with claim l comprising an annular non-magnetic member seated upon said spacer and having a ange portion extending between said armature and said magnetic member and spaced from said armature.

3. An electroacoustic transducer comprising a magnetic circuit including a permanent magnet and having coaxial, inner and outer annular pole faces, an annular magnetic spacer seated upon the outer pole face, an annular magnetic armature seated adjacent its periphery upon said spacer and having its inner edge portion in juxtaposition to the inner pole face, a nonmagnetic spacer seated upon said rst spacer, an annular magnetic member overlying said armature and seated upon said second spacer, and an annular magnetic member interposed between said first magnetic member and said outer pole face.

EDWARD E. MOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,331,469 Kraft Feb. 17, 1920 1,439,785 Andrew Dec. 26, 1922 1,455,745 Egerton May 15, 1923 1,708,943 Goodrum Apr. 16, 1929 1,800,573 Skala Apr. 14, 1931 1,830,160 French Nov.. 3, 1931 2,003,908 Smith June 4, 1935 2,044,608 Harford June 16, 1936 2,077,425- Lieber Apr. 20, 1937 2,141,595 Cornwell Dec. 27, 1938 2,170,571 Mott Aug. 22, 1939 2,249,160 Mott July 15, 1941 2,360,796 Roberton Oct. 17, 1944 2,395,166 Collins Feb. 19, 1946 2,404,727 Houtz July 23, 1946 

